I am rebuilding my heater box and core for my JH and was able to get everything back into the car. I hooked everything back up and was not able to get the heater bypass valve to operate when the control lever is set to cold. It seems the vacuum is not sufficient by the time it gets from the manifild to the control valve on the heater box then back to the bypass valve that is bolted to the valve cover.

At least the default for the bypass is in the open position so hot coolant is always passing through the heater core but it would be nice to have the bypass operational for those hot summer days when I don't beed another radiant heat source in the cockpit. The bypass valve moves freely by hand and does close off the flow, I'm wondering if the control vacuum valve is not working properly when depressed by the control lever or my vacuum lines are toast. I tried to get new vacuum line at Napa but they don't have anything even close. Anybody ran into this before?

Well, in all of the dozen or so JH's I have encountered or worked on, the metal vacuum operated valve which is attached to the valve cover was rusted open and totally ruined. I found a generic, mostly plastic, valve at NAPA that worked very well. The vcuum comes through the manifold t near the brake booster, goes inside the car to a tiny valve mounted in the footwell on the side next to the gas pedal on the heater box. Looks like a square click on click off ball point pen. When the lever is thrown all the way over, part of the mechanism pushes down on the tiny valve end and opens the vacuum or closes it off...I cannot remember. I suspect the big valve in the engine compartment is closed on vacuum and opened without...nonetheless, that is the wayit works. You will have to be a contortionist to get up in there in the drivers footwell to see it, but if either of the small rubber hoses is off or broken, the valve cannot work. I suggest removing the valve mounted on the engine and inspecting it. If it is rusted badly or will not actuate with and without a bit of vacuum, it should be replaced. Some use an aftermarket cable operated off/on weater valve, with the handle up undeer the dash mounted on the left side wall of the car for access...but in the later cars, I suspect the owner has overlooked this vacuum switch up inside that I mentioned. In case the hoses to this tiny switch are bad, you will have a vacuum leak that is continuous in your system...that is not desireable. Check with our webmaster, Greg, to see if he has replacement water valves..if not, try Delta. I have no financial affilliation with either...both great helpful people.

One more thing...you mentioned you had weak vacuum to the water valve mounted on the engine. Either one of the lines from the T which goes inside the car to the mini control valve is not connected or leaking...or the one that goes to the water valve is not connected, or leaking...if all is right you have a bit of vacuum...enough to close the water valve. Doesn't take much. If you hve a vacuum gauge, connect it to the line giong to the water valve, and check what you get when the control lever is moved from one end of its travel to the other. Off should be no vacuum at all and on shoudl produce a couple of pounds. You can also take a hand operated vacuum pump and connect directly to the water valve,, a few pumps and it should operate...if not, it is defective. Hope all this helps...didn't mean to go into such detail...but that is the way I would tell you if I was there in your shop with you !!!!!
Good luck and let us know what you find.

OK, reading your post...you say the valve works by hand...good to knnw. put the vacuum pump on it and see if that works it. If it does...check the rubber small tube going in the car and the return tube coming out to the water valve...remember what I said about the switch that opens and closes these two lines that is mounted up on the side of your heater box above your accellerator....

When you had it apart, did you notice it? Does it work? Does the control lever when moved to the far end of its travel actuation the mini plunger in this switch?

As far as the mini switch, I did remove it before sandblasting the box and cleaned it out. I have the baffles adjusted so that the cold setting in fact does depress the plunger so all is good there. More good news is that I hooked up the vacuum line directly to the bypass water valve and it immediately opens and moves freeley.

I think I have a bad line either from the vacuum port to the mini valve or from the mini valve to the bypass valve so if I can find the correct size lines at an auto store I should be good to go. Thanks again for your help

Those few people who may have been following this, I finally got the system to work. As it turned out I wasn't able to find any 1/4" vacuum tubing at any parts stores but by cutting 3/4" or so off each end the lines I was able to get a nice seal when pushed onto the pipes on the mini valve and bypass valve. After doing so I was able to get the mini valve to close when indicator moved to COLD, of course the cable needs to be adjusted to the mini valve plunger depressed all the way when closed.

Another thing I noticed is that if the hoses attached to the mini valve are reversed, the valve will close the bypass valve when depressed but will not release the bypass valve (ie open flow) when the selector is removed from the COLD setting. With the engine very hot and the cold setting on, theheater box has next to no heat coming to it, very nice, worth the time and effort to getting it going.

Frank
I noticed that all 1/4 in. vacuum hoses going to/from the heater system are missing. I understand the regulating control valve is mounted above the valve cover in the engine bay, this valve is activated by a small on/off vacuum switch located on the left side of the heater box inside the car. This little valve has 2 ports i assume one port is for vacuum coming from the intake manifold, and the other port going to the heater control valve in the engine compartment.
Can you please confirm that is how it works. Thanks
Francis G

Yes, that is partially correct...however I have seen some early Jensen Healeys that did not have that mysterious little valve at all.... One line from the vacuum goes to the magic plunger switch..the other goes to the water valve, vacuum operated. The valve works like this: with the heater turned off, the vacuum coming into the switch is actually going to the vacuum operated water valve. When the switch is turned off, the vacuum going to the water valve is on...kinda wierd. Bruce Madden in Atlanta told me he had some new old stock of this water valve...

So..when the engine is running there is vacuum to the valve via the magic gimmick and when you shut the engine down you can actually see the valve open as the vacuum is gone. Then when you start it up, the vacuum closes the valve and you get no heat...turn on the heat and the vacuum is released and the valve opens...one would think it should be the other way around..but it is not.
Many JHs have this water valve and it is often rusted so bad that it is useless...I should add.

Well, this might explain why I can't seem to get the "cold" setting to work. Always seems like Ms. Jenavieve is blowing heat.

Being that I can't imagine ever wanting heat (she's a "nice summer day" sort of car, if it's cold out, I drive the car that's designed for winter) one of these days I'll probably look into just bypassing the heating system entirely. Maybe just splice the source and return lines to the heater box, disconnecting them entirely. Unless that's going to play havoc with the cooling system.

The heater does add some cooling capacity when water if flowing through it. If the cooling system is working correctly, it should not be necessary.

My car has the same problem of hot air in the cockpit even though the vacuum-operated shut-off valve seems to be working correctly. I am not sure if the problem is water flow into the heater core or if there is just heat getting in through the fire wall. Maybe there is water leaking through the old valve even though it is "off". I am tempted to splice a manual shut-off valve into the line to the heater to see if that changes anything. This is going to happen later rather than sooner since there are still a lot of higher-priority things on my list.